Many utility lines, pipelines and other underground components are installed in or under the ground by boring a borehole in a generally-horizontal direction in the ground rather than by digging a trench. This type of construction, which is sometimes referred to as “horizontal boring”, “directional drilling” or “horizontal directional drilling”, reduces the need to dig a trench in order to install an underground component, and thereby saves several steps in the installation process. If no trench is dug, there will be no trench to fill, and no disturbed surface to reclaim. A directional drilling machine may be operated to drill a bore along a planned path underground. Typically, the planned path is generally arcuate in shape from the entry point at the surface of the ground, continuing underneath a roadway, river or other obstacle, to an exit point on the surface on the other side of the obstacle.
A typical directional drilling machine includes a thrust frame that can be aligned at an oblique angle with respect to the ground. Mounted on a drive carriage on the thrust frame is a pipe-rotation mechanism that is adapted to rotate and thrust or retract a series of interconnected pipe sections commonly referred to as a drill string). The drive carriage also includes a carriage drive assembly that is adapted to push the carriage along the thrust frame. The combination of rotation of the drill string and longitudinal movement of the drive carriage along the thrust frame causes the drill string to be advanced into or withdrawn from the ground.
To drill an original or pilot bore using a directional drilling machine, the thrust frame is oriented at an angle relative to the ground, and the drive carriage is retracted to an upper end of the frame. A drill pipe section is coupled to the pipe-rotation mechanism on the drive carriage. A boring tool or cutting head is mounted to the terminal end of the pipe, and the drive carriage is driven in a downward direction along the inclined thrust frame. As the drive carriage is driven downwardly, the pipe-rotation mechanism rotates the pipe about the boring axis, thereby causing the pipe (with boring tool mounted thereon) to be thrust into the ground to drill or bore a hole.
As the drilling operation proceeds, the drill string is lengthened by adding pipe sections to the string. The pipe sections may be provided with a male threaded connector on one end and a female threaded connector on the other end. Each time a pipe section is added to the drill string, the pipe section being added is aligned with the drill string and the threaded connector on its far end is mated with the threaded connector on the near end of the drill string. Generally, the drill string is restrained against rotation while the pipe being added is rotated to engage the threaded connector on the far end of the pipe section with the threaded connector on the near end of the drill string to create a threaded connection between the components.
Hydraulically actuated wrenches are typically mounted on the horizontal directional drilling rig may be used to hold the drill string as pipe sections are added to lengthen the drill string. These wrenches are also used to separate pipe sections and typically comprise two pairs of opposed jaws, one for the male-threaded pipe and the other for the female-threaded pipe of the adjacent components of the drill string. Each pair of jaws is adapted to clamp around a pipe section, one on the far side and the other on the near side of the threaded connection. At least one pair of jaws of the wrench assembly will pivot with respect to the other pair of jaws to twist one of the pipe sections with respect to the other. However, one skilled in the art will appreciate that connecting the pipe sections may be accomplished using one wrench and the rotating drive of the HDD machine.
When the boring tool reaches a desired depth during the drilling operation, it can be directed along a generally horizontal path and back up to break the surface of the ground at a distant exit point. To control the direction of the borehole, a boring tool with an angled-face or a deflection member may be used. When the direction of the borehole must be changed, the boring tool is positioned with the angled-face or deflection member oriented to cause the tool to move in the desired direction. This ability to change the direction of travel of the drill string also allows the operator to steer the drill string around underground obstacles like large roots and rocks.
When the pilot bore is complete, the boring tool is removed from the second end of the drill string, and the pipe sections are disconnected from each other to disassemble the drill string on the exit side of the bore. In the alternative, the bore may be enlarged by replacing the boring tool with an enlarging device, commonly known as a backreamer. If a backreamer is used, it will be connected to the far or distal end of the drill string in place of the boring tool and moved through the pilot bore back towards the boring machine, either with or without rotation of the drill string. The backreamer expands and stabilizes the walls of the bore, generally while pulling a product pipe or other underground component through the enlarged bore behind it. Movement of the backreamer back towards the drilling machine is accomplished by driving the drive carriage in a rearward direction on the thrust frame to withdraw a pipe section, disconnecting the withdrawn pipe section from the drill string, connecting the next pipe section remaining in the drill string to the pipe rotation mechanism on the drive carriage and repeating the process until all of the pipe sections have been withdrawn from the ground. Each pipe section in the drill string may be uncoupled from the drill string using the same wrench assembly that was used to connect the drill pipes when the pilot bore was drilled. The disconnected pipe section is placed in a stack or loaded into a pipe section magazine of the directional drilling machine.
There are several operations that must be performed on the exit side of the bore where the drill string emerges from the ground. For example, the boring tool may be disconnected from the end of the drill string and the pipe sections of the drill string may be disconnected one by one from the drill string. If a backreamer is used, it may be installed in place of the boring tool. High torque is typically used in order to loosen the boring tool or a pipe section for removal from the drill string or to install the backreamer on the drill string. Most commonly, the drill crew will use a pair of large wrenches such as pipe wrenches or oil field tongs to remove the boring tool and each pipe section, or to install a backreamer. Frequently, the drill crew will connect the handle of the wrench to the bucket of a hydraulic excavator using a chain or strap, and then use the excavator to apply a vertical force to the bucket while the drilling rig operator rotates the drill string to loosen the boring tool or a pipe section or to tighten the backreamer on the end of the drill string. If the drill string is to be disassembled on the exit side, the individual pipe sections may be placed in a stack or in a pipe section magazine. These pipe sections are heavy and long, and it is labor-intensive to disconnect them manually on the exit side of the drill site.